Micro-bead blasting process for removing a silicone flash layer

ABSTRACT

Using compression molding to form lenses over LED arrays on a metal core printed circuit board leaves a flash layer of silicone covering the contact pads that are later required to connect the arrays to power. A method for removing the flash layer involves blasting particles of sodium bicarbonate at the flash layer. A nozzle is positioned within thirty millimeters of the top surface of the flash layer. The stream of air that exits from the nozzle is directed towards the top surface at an angle between five and thirty degrees away from normal to the top surface. The particles of sodium bicarbonate are added to the stream of air and then collide into the top surface of the silicone flash layer until the flash layer laterally above the contact pads is removed. The edge of silicone around the cleaned contact pad thereafter contains a trace amount of sodium bicarbonate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.13/304,769 entitled “Micro-Bead Blasting Process for Removing a SiliconeFlash Layer,” filed on Nov. 28, 2011, the subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to packaging light-emittingdiodes and, more specifically, to a method for removing a silicone flashlayer following compression molding.

BACKGROUND INFORMATION

A light emitting diode (LED) is a solid state device that convertselectrical energy to light. Light is emitted from active layers ofsemiconductor material sandwiched between oppositely doped layers when avoltage is applied across the doped layers. In order to use an LED chip,the chip is typically enclosed in a package that focuses the light andthat protects the chip from being damaged. When LEDs are packages inarrays as opposed to as discrete light emitters, the LED chips of thearrays are mounted directed on a printed circuit board without thecarrier substrate conventionally used with discrete light emitters. TheLED chips packaged as arrays are electrically connected to contact padsand to traces in a top trace layer of the printed circuit board. The LEDchips are wire bonded to the traces on the top side of the printedcircuit board. The printed circuit board is then segmented to formdiscrete array light sources. Larger exposed areas of the traces on thetop side form contact pads to which supply power is connected to eachdiscrete array light source.

The LEDs are typically covered with a layer of phosphor before the arraylight sources on the printed circuit board are segmented. The phosphorconverts a portion of the blue light generated by the LEDs to light inthe yellow region of the optical spectrum. The combination of the blueand yellow light is perceived as “white” light by a human observer.Before the array light sources are segmented, the LEDs are typicallycovered by a layer of silicone that is formed into a lens above eachlight source. The layer of silicone also protects the LED chips andtop-side wire bonds.

A slurry containing the phosphor has been conventionally dispensedmanually into a ring or dam around the LED chips of each array lightsource. Then injection molding or casting molding has been used to forma lens above each array light source. The manufacturing process for LEDlight sources has been improved by combining the steps of dispensing thephosphor and forming the lens. By adding the phosphor to the silicone,the separate step of dispensing phosphor can be eliminated, and lensesare formed with phosphor dispersed throughout each lens. The lenses areformed using injection molding in which lens cavities that contain theLED dies are filled with the lens material, and the excess lens materialis squeezed out of a leakage path.

When casting molding is used, a phosphor silicone slurry is firstdispensed into the bottom half of each cavity, and then the top half ofthe cavity closes to define the lens structure and squeezes out theexcessive lens material. The injection molding and casting moldingprocesses have multiple disadvantages. First, the phosphor and thesilicone are expensive, and the lens material that is squeezed out ofthe cavities is wasted. Second, the quality of the lenses formed withinjection molding and casting molding is low because bubbles andnonuniformities remain in the finished product.

These disadvantages can be overcome by using compression molding inwhich the lens material is contained in a single sealed cavity undercompression. Placing the lens material initially under a vacuum and thenunder high pressure between the two parts of the compression moldensures a uniform consistency of the lens material throughout the cavityand prevents bubbles from forming. Moreover, only the amount of lensmaterial that is actually used is pumped into the sealed cavity, so nolens material is wasted.

Unfortunately, at least one complication must be overcome beforecompression molding can be used to form lenses over LED array lightsources on printed circuit boards. The compression molding processrelies on the lens material being able to flow freely through a flashlayer between the individual lens cavities so that the lens material isuniformly distributed. Consequently, the entire surface of the printedcircuit board between the lenses is covered by the flash layer. So thecontact pads for each LED array on the top side of the printed circuitboard are covered by the flash layer of silicone, which inhibits anelectrical contact being made with the contact pads. Existingcompression molding techniques require the flash layer to be at leastfifty microns thick, whereas the trace layer that forms the contact padscan be as thin as a couple of microns. Manually scraping off the flashlayer would either damage the contact pads or not remove the siliconefrom the entire surface of the contact pads. Taping over the contactpads before the compression molding step and then later removing thetape would increase the cost by adding two additional steps. Inaddition, the silicone at the edges of the lenses could tear when thetape is lifted.

An efficient method is sought for removing the flash layer of siliconethat results from compression molding without damaging either the lensesor the trace metal layer that forms the contact pads on the top side ofthe printed circuit board.

SUMMARY

Using compression molding to form lenses of silicone over LED arrays ona metal core printed circuit board (MCPCB) leaves a flash layer ofsilicone covering the contact pads that are later required to connectthe arrays to power. A method for removing the silicone flash layerinvolves blasting abrasive particles in a stream of air at the siliconeflash layer. The particles can be made of sodium bicarbonate, sodiumsulfate, ammonium bicarbonate, silicon dioxide, aluminum oxide, aplastic or glass beads. The abrasive particles have a median diameterthat is between forty and sixty microns. A nozzle is positioned withinthirty millimeters of the top surface of the flash layer. The flow ofair is generated by compressing the air to a pressure of more than onehundred pounds per square inch and allowing the compressed air to escapefrom a nozzle that has a diameter of less than two millimeters. Thestream of air that exits from the nozzle is directed towards the topsurface at an angle between five and thirty degrees away from normal tothe top surface. The abrasive particles are added to the stream of airsuch that the particles are carried by the stream of air. The particlesthen collide into the top surface of the flash layer of silicone untilthe flash layer laterally above the contact pads is removed.

An LED array light source includes LED dies mounted on a MCPCB. A layerof silicone forms a lens above the LED dies. The MCPCB has a trace layerand a solder mask layer. The LED dies are electrically coupled to thetrace layer. The solder mask layer is disposed over the trace layer. Acontact pad is formed on the trace layer by an opening in the soldermask. The layer of silicone that is disposed over the LED dies forms anedge around the contact pad. The layer of silicone is not presentlaterally above the contact pad. The layer of silicone contains a traceamount of a blasting medium at the edge of the layer of silicone. Theblasting medium is sodium bicarbonate, sodium sulfate or ammoniumbicarbonate. The layer of silicone can also contain phosphor. The traceamount of the blasting medium was embedded into the edge of the siliconearound the contact pad when a flash layer of silicone was removed fromabove the contact pad by blasting abrasive particles of the blastingmedium in a stream of air at the silicone flash layer.

In another embodiment, an LED array light source includes a printedcircuit board (PCB), an LED die, a contact pad and a layer of silicone.The PCB has a top side, a bottom side, and four top edges. The LED dieand the contact pad are disposed on the top side of the PCB. The layerof silicone is disposed over the LED die and extends to each top edge ofthe PCB. However, the layer of silicone is not disposed laterally abovea portion of the contact pad because the silicone has been removed byblasting abrasive particles in a stream of air at the layer of silicone.

In yet another embodiment, a high-pressure stream of water is used toremove the flash layer of silicone over the contact pads. The water ispressurized to a pressure of over fifty pounds per square inch and thenforced through a nozzle with a diameter of less than one millimeter. Thepressurized stream of water is aimed directly at the silicone flashlayer over the contact pads until the flash layer is removed.Alternatively to using pure water, abrasive particles made of silica,aluminum oxide or garnet can be added to the stream of water to allowthe deflashing process to be performed at a lower water pressurecompared to using pure water.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a top view of a metal core printed circuit board (MCPCB) onwhich multiple arrays of LED dies are mounted.

FIG. 2 is a top view of the MCPCB of FIG. 1 on which areas have beenmarked to show where a flash layer should be removed to expose contactpads.

FIG. 3 is a cross sectional view of the MCPCB of FIG. 1 showing theflash layer that is to be removed using the novel blasting process.

FIG. 4 is a more detailed view of the flash layer of FIG. 3.

FIG. 5 is a flowchart of steps of a method for removing a flash layer ofsilicone that covers contact pads without damaging the contact pads.

FIG. 6 is a cross sectional view illustrating blasting particlescolliding with a flash layer at a blasting site enclosed by a blastingmask.

FIG. 7 is a cross sectional view of the blasting sites of FIG. 3 afterthe flash layer has been removed using the method of FIG. 5.

FIG. 8 is a top view of another MCPCB from which a flash layer ofsilicone is to be removed using the method of FIG. 5.

FIG. 9 is a top-down perspective view of a blasting site between fourlenses on the MCPCB of FIG. 8.

FIG. 10 is a perspective view of a discrete light source with top-sideelectrical contacts from which a flash silicone layer has been removed.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a top view of a metal-core printed circuit board (MCPCB) 10 onwhich multiple arrays of LED dies 11 are mounted. Because MCPCB 10 has ametal core, it would be difficult to supply power to the LED dies 11through through-hole vias that pass from the LEDs through the printedcircuit board to the bottom surface of the board. Consequently, the LEDdies 11 are electrically connected to contact pads on the top side ofMCPCB 10. The MCPCB 10 is then segmented to form discrete array lightsources. The discrete light sources can be used as standardized photonbuilding blocks by packaging them in a multitude of ways using a moldedinterconnect structure that electrically contacts the photon buildingblocks from the top side. For a more detailed description of discretelight sources packaged in an interconnect structure that electricallyconnects to the discrete light sources only from the top side, see U.S.patent application Ser. No. 12/987,148 filed on Jan. 9, 2011, entitled“Packaging Photon Building Blocks Having Only Top Side Connections in anInterconnect Structure,” published as U.S. Pat. App. Pub. 2012/0175643,which is incorporated herein by reference.

In the embodiment of FIG. 1, MCPCB 10 includes a 5×12 matrix of 4×4 LEDarrays. MCPCB 10 has a length of about 250 mm and a width of about 75mm. Each LED array is later segmented into a square of the MCPCB that is11.5 mm on a side. Thus, MCPCB 10 has a very high density of lightsources per area of the printed circuit board. There are less than threemillimeters of space on the board between the edge of the lens thatcovers the LED dies 11 and the edge of each of the segmented squarelight source. At each corner of the square is a contact pad 12 that isused to supply power to the array light source. The contact pads 12 areformed by exposing large triangular areas of a trace layer. The tracelayer is covered by a solder mask layer 13 of hardened epoxy. Holes insolder mask layer 13 form the contact pads 12 and the locations on thetrace layer below to which the LED dies 11 are wire bonded.

A lens is formed over each LED array using compression molding.Compression molding can be used because there are no holes or openingfrom the top side to the bottom side of MCPCB 10 through which highpressure molding material could escape. Conventional printed circuitboards used to mount LED arrays have punch-outs or etchings cuts toisolate the electrical leads of each LED array. MCPCB 10, on the otherhand, is a closed board with no punch outs, holes or etching cutsthrough the board. The very high density of components and the closedboard of MCPCB 10 are conducive to compression molding. A single moldingchamber is formed over the top of MCPCB 10 by sealing the chamber aroundthe border 14 of MCPCB 10. A small space is maintained between soldermask layer 13 and the mold cover to allow the molding material to flowfreely between the individual cavities above the LED arrays. In theactual molding process, MCPCB 10 is inverted and lowered into the moldcover, which contains the lens cavities. The molding material is pumpedinto the single molding chamber under pressure and fills all of thecrevices of the cavities without leaving bubbles or nonuniformities inthe hardened molding material. The molding material that fills the smallspace between the mold cover and solder mask layer 13 forms a thin flashlayer that covers the contact pads 12 that must later be electricallycoupled to the interconnect packaging structure.

In one embodiment, the molding material is a slurry of phosphorparticles in silicone. The phosphor is evenly dispersed throughout thesilicone and converts a portion of the blue light generated by the LEDsinto light in the yellow and red regions of the optical spectrum. Theblue light from the LEDs and the yellow and red light from the phosphorcombine to yield white light, which is optically spread out by thesurface of the lens. After the lenses are formed using compressionmolding, the individual LED array light sources are segmented by cuttingMCPCB 10 into squares. It is more efficient, however, first to removethe flash layer that covers the contact pads 12 before segmenting MCPCB10 into individual LED array light sources.

FIG. 2 is a top view of MCPCB 10 of FIG. 1 on which areas have beenmarked to show where the flash layer should be removed to expose thecontact pads 12. For a unit size of 11.5 mm by 11.5 mm for the LED arraylight sources of FIG. 1, the contact pads 12 can be cleaned of thesilicone flash layer by removing silicone from 5 mm by 5 mm squares. Anovel micro-bead blasting process is used to remove the silicone flashlayer from the square blasting sites 15.

FIG. 3 is a cross sectional view of MCPCB 10 of FIG. 1 showing the flashlayer 16 that is to be removed using the novel blasting process. MCPCB10 has a thick solid aluminum base 17. For example, aluminum base 17 is1.6 mm thick. A dielectric layer 18 separates aluminum base 17 from thetrace layer 19 that forms the contact pads 12. Dielectric layer 18 has athickness of about twenty microns (micrometers or μm). Trace layer 19does not entirely cover dielectric layer 18, but rather is formed bypatterned traces separated by dielectric material. Solder mask layer 13covers trace layer 19 and has openings only over the contact pads 12 andthe locations at which the LED dies 11 are wire bonded to traces.

The molded silicone forms lenses 20 over the arrays of LED dies 11. Inthe embodiment of FIG. 3, the diameter of lens 20 is about twice as longas each side of the 4×4 array of LED dies so as to allow most of theemitted light to reach the surface of lens 20 within the critical anglerequired for the light to escape from the lens. The height of the lens20 is about 1.5 mm from solder mask layer 13. Other embodiments havelenses of different sizes and shapes over the LED dies 11. For example,the silicone above each LED array can have a small overall curvaturethat is covered by many smaller micro-structures, such as hemispheres orpyramids. Alternatively, the lens shape can have a dimple above themiddle of each LED array.

FIG. 4 is a more detailed view of flash layer 16 of FIG. 3. FIG. 4 showsthat flash layer 16 is relatively thick compared to trace layer 19.Whereas in some compression molding processes flash layer 16 has athickness between fifty to one hundred microns, trace layer 19 can havea thickness of less than five microns. Trace layer 19 typically hasthree sublayers: a thicker lower layer of copper, a thinner middle layerof nickel, and a thinner upper layer of either gold or silver. Copper isless expensive than nickel, gold or silver, so the traces are comprisedmostly of copper. The upper layer of gold or silver are required becauseit is difficult to solder the wire bonds directly to copper. The middlelayer of nickel is used to attach the gold or silver to the thickercopper layer because gold and silver do not readily adhere directly tocopper. The copper is typically 2-80 microns thick, the nickel istypically 1-3 microns thick, and the gold or silver is typically 1-5microns thick. Thus, the contact pads 12 will be damaged if the gold orsilver that is no thicker than five microns is removed from the uppersurface of the trace layer 19. The novel micro-bead blasting processprovides a way of removing silicone flash layer that is about fiftymicrons thick without removing the upper layer of trace layer 19, whichis only about one tenth as thick.

FIG. 5 is a flowchart illustrating steps 21-26 of a micro-bead blastingprocess that removes a flash layer of silicone that covers contact padswithout damaging the contact pads. The steps of the method of FIG. 5 aredescribed in relation to FIG. 4.

In a first step 21, the flash layer 16 is formed over the printedcircuit board 10 using compression molding. Although the flash layer 16of FIG. 4 results from compression molding silicone, other transparentmolding materials may also be used, such as epoxy. The flash layer ofsilicone 16 in FIG. 4 is disposed above two contact pads 12.

In step 22, a nozzle 27 is positioned within thirty millimeters of a topsurface 28 of flash layer 16. In order to clean a blasting site 15 thatis 5 mm by 5 mm square, the method of FIG. 5 uses a nozzle 27 that has adiameter of about two millimeters and that is placed about twenty-twomillimeters above top surface 28. A smaller nozzle diameter would beused to remove a flash layer from a smaller blasting site, in which casethe nozzle would be positioned closer to the top surface of the flashlayer. For example, in order to clean the flash layer from a blastingsite 15 having a diameter of two millimeters located between LED arrayshaving unit sizes of five millimeters on a side, nozzle 27 would have adiameter of about 0.5 millimeters and would be positioned about twomillimeters above the top surface 28 of flash layer 16. The blastingsite is located over the contact pads 12 that are to be cleaned of flashlayer 16. Positioning nozzle 27 farther away from top surface 28 allowsthe stream of air exiting the nozzle to spread out into a wider plume 29before contacting top surface 28. Thus, nozzle 27 must be positionedcloser to top surface 28 in order to maintain the stream of air within asmaller blasting site 15.

In step 23, the flow of air that exits nozzle 27 is directed at topsurface 28 of flash layer 16 within blasting site 15. The stream of airthat exits from nozzle 27 is directed towards top surface 28 at an anglethat is between five and thirty degrees away from a normal angle to thetop surface. The stream of air is generated by compressing the air to apressure of more than one hundred pounds per square inch (psi) and thenallowing the compressed air to escape from nozzle 27. In order to cleana blasting site 15 that is 5 mm by 5 mm square, the flow of air isgenerated by compressing the air to a pressure between one hundred andone hundred forty pounds per square inch and then allowing thecompressed air to escape from a nozzle that has a diameter of less thantwo millimeters.

In step 24, blasting particles 30 of a blasting medium are added to thestream of air such that the particles are carried by the stream of airand collide into top surface 28 of flash layer 16 above contact pad 12.The blasting particles 30 are also called micro beads, although theyneed not be spherically shaped. The blasting medium should have a Mohshardness of less than three; sodium bicarbonate (NaHCO₃), sodium sulfateand ammonium bicarbonate (ammonium hydrogen carbonate, (NH₄HCO₃)) can beused. In one embodiment, the blasting particles 30 are monoclinic prismsof sodium bicarbonate that have been purified and sorted through a sieveto have a uniform particle size of about fifty microns in the longestdimension. The blasting particles 30 are stored as a powder and areadded into the flow of air by a mixer 31 shortly before exiting nozzle27.

When cleaning a blasting site 15 that is 5 mm by 5 mm square, the nozzlecan be placed about twenty-two millimeters above top surface 28, whichallows the blasting particles 30 to achieve their highest velocity. Whenthe particles 30 are first added to the flow of air by mixer 31, theinertia of the particles prevents them from immediately accelerating tothe speed of the air flow. However, within about twenty-two millimetersthe particles 30 have accelerated to the speed of the stream of air andhave achieved their highest velocity. At distances greater than aboutthirty millimeters from nozzle 27, resistance from ambient air overcomesthe thrust from the stream of air and slows down the particles 30. Atdistances less than about twenty millimeters from nozzle 27, theparticles 30 have not yet accelerated to the speed of the flow of air.Thus, where particles of about fifty microns in length are used, flashlayer 16 can be removed in the shortest period of time by blasting theparticles from a distance of about twenty-two millimeters because theparticles possess the most amount of kinetic energy at that distancefrom the nozzle.

In step 25, the particles 30 are collided into flash layer 16 until theflash layer laterally above contact pad 12 is removed. The particles 30have facets and edges that rip the silicone of flash layer 16 apart.Then the air blows the ripped pieces of silicone away. Small amounts ofsodium bicarbonate remain embedded in the silicone that has not beenremoved. When cleaning the relatively large blasting sites 15 of FIG. 2,nozzle 27 may be placed at about twenty-two millimeters from top surface28 of flash layer 16, which permits the particles 30 to acquire theirmaximum kinetic energy. Consequently, the flash layer in the blastingsites 15 that are squares 5 mm on a side can be removed in a relativelyshort 2-3 seconds. On the other hand, when cleaning the relatively smallblasting site 15 having a diameter of two millimeters located betweenLED arrays having unit sizes of five millimeters on a side, nozzle 27must be placed a relatively close two millimeters from top surface 28,which does not permit the particles 30 to achieve their maximum speed.Consequently, the flash layer in a blasting site with a diameter of twomillimeters can be removed only after a relatively long eight seconds ofblasting.

The stream of air exiting nozzle 27 is not directed in step 23 towardsflash layer 16 at an angle normal to top surface 28, i.e., the stream ofair is not directed orthogonally to top surface 28. Instead, the streamof air is directed towards flash layer 16 at an angle that is betweenfive and thirty degrees away from normal to the top surface in order topermit the particles 30 to be blown away from the blasting site. If thenozzle were to be directed orthogonally to the top surface of the flashlayer, the blasting particles would bounce straight back up and collidewith the particles in the stream of air. This would reduce the force bywhich the blasting particles collide with the flash layer. In addition,the particles would not bounce sideways after striking the top surfaceand therefore would not be carried out of the blasting site and wouldbuild up. On the other hand, if the nozzle were directed at an shallowangle to the top surface of the flash layer, such as an angle greaterthan thirty degrees from normal, then the vector of the particle speednormal to the top surface would be insufficient to remove the flashlayer. The particles would tend to be deflected by the top surface andwould not break into the surface.

Even at a steeper angle of incidence, such as ten degrees, the blastingparticles 30 are more likely to bounce off of top surface 28 instead ofbreaking into the surface when flash layer 16 is thicker. At thebeginning of the blasting process when flash layer 16 is still aboutfifty microns thick, the particles 30 are more likely to bounce off topsurface 28 because the thicker silicone flash layer can elasticallycompress to absorb the impact of the particles. As flash layer 16 iseaten away and becomes thinner, the rate of silicone removal becomesfaster as the kinetic energy of the particles increasingly tears thesilicone as opposed to being absorbed by the silicone.

Current compression molding techniques specify that the thickness of aflash layer of silicone can be 50±25% microns. It is desirable to keepthe flash layer as thin as possible to save on silicone but yet allowthe silicone to flow freely between the individual lens cavities toachieve high quality lens structures. Where the flash layer is thinnerthan thirty microns, the elasticity of the silicone layer is reduced tothe point that blasting particles do not readily bounce off of thesilicone but rather tear the silicone. As even thinner flash layersbecome possible, the flow of compressed air alone will be sufficient toremove the flash layer from between the lens structures.

FIG. 6 illustrates blasting particles at a blasting site that isenclosed by a blasting mask 31. Blasting mask 31 is made of stainlesssteel and is about 200-500 microns thick. Mask 31 is used when thelenses are particularly close to the blasting sites 15 and must beprotected from the blasting particles 30. For example, blasting mask 31is used for blasting sites located between LED arrays having unit sizesof five millimeters on a side. The blasting process is speeded up byusing a mask because the flow of air need not be turned off when movingfrom site to site. Each lens 20 is protected from the blasting particles30 by mask 31 as the stream of air moves over the lens to a new blastingsite. In contrast, where no blasting mask is used with larger unitsizes, such as an array unit size of 11.5 mm on a side, the flow of airis turned off as the position of the nozzle is moved from one blastingsite to another in order to avoid damaging the lens structures.

Using a blasting mask, however, creates other complications that slowdown the blasting process. The thickness of the blasting mask creates awell that both (i) obstructs the corners of the blasting site from beingreached by the stream of air and (ii) hinders the blasting particlesfrom being blown away from the blasting site. First, the blasting maskobstructs the nearest corner of the blasting site from direct blastingby the angled stream of air. Thus, the far side of the blasting site 15is cleaned first, and then MCPCB 10 is rotated to permit the cleaning ofthe other side of the blasting site. The rotation and double pass of thestream of air slow the blasting process. Second, the sides 32 of theblasting mask 31 form a deep well that tends to trap the blastingparticles 30. If blasting particles 30 from the stream of air collidewith other particles that previously accumulated over the surface of theblasting site 15, then the silicone flash layer 16 will not be torn andultimately removed. Thus, the angle of the stream of air is increasedtowards thirty degrees from normal to top surface 28 in order to bouncethe particles 30 away from the incoming particles and out of the well.In addition, the pressure of the air used to generate the stream of airis increased towards one hundred forty pounds per square inch in orderto provide the particles with enough kinetic energy to bounce out of thewell.

FIG. 7 is a cross sectional view of the blasting sites 15 of FIG. 3after the flash layers 16 have been removed using the method of FIG. 5.FIG. 7 shows that after the blasting process, the layer of siliconeforms an edge 33 around the contact pads 12 that have been cleaned. Someof the blasting particles 30 break apart in the blasting process andform dust having particles sizes much smaller than 50 microns. Some ofthe dust lodges in the silicone around the blasting sites 15. Thus, thesilicone at edge 33 contains a trace amount of the blasting medium, suchas sodium bicarbonate, that remains from the blasting particles 30. Thetrace amount of sodium bicarbonate can be detected in the segmented LEDarray light sources using a gas spectrometer.

FIG. 8 is a top view of another MCPCB 34 from which a flash layer ofsilicone is removed using the method of FIG. 5. In FIG. 8, the lenses 35and flash layer 36 have already been formed by compression molding. LikeMCPCB 10 of FIG. 1, MCPCB 34 also includes a 5×12 matrix of LED arrays.And each LED array is later segmented into a square of the MCPCB that is11.5 mm on a side. Unlike MCPCB 10 of FIG. 1, however, the contact pads37 on MCPCB 34 are not formed by exposing areas of a trace layer that iscovered by a solder mask layer. Instead, the contact pads 37 are fourstrips of metal that extend out from under each lens 35. The flash layer36 covers the metal strips.

FIG. 9 is a top-down perspective view of a blasting site 38 between fourlenses 35 on MCPCB 34. The micro-bead blasting process was performedusing a 0.077 inch diameter nozzle positioned about 22 millimeters aboveflash layer 36. A pressure of 120 psi was used to generate the stream ofair that contained particles of sodium bicarbonate having a mediandiameter of about 50 microns. The stream of air containing the blastingparticles was blasted at blasting site 38 for 1.65 seconds. The blastingremoved material to various degrees progressing outwards from the centerof blasting site 38. At the center of blasting site 38, the entirethickness of flash layer 36 has been removed, and the blasting has evenremoved some of the upper layer of gold from the contact pads 37. Someof the dielectric layer was also removed from the center of blastingsite 38. Moving outwards from the center of blasting site 38, only thesilicone was removed from a large portion of the contacts pads 37without damaging the upper layers of the contact pads 37. This region ismarked with diagonal hashes in FIG. 9. In the next region on eachcontact pad 37 outwards from the center to the blasting site, thesilicone flash layer 36 was not entirely removed from the contact pad.FIG. 9 shows areas 39 on the corners of lenses 35 that have beenpartially roughened by the blasting process.

FIG. 10 is a perspective view of a discrete light source 40 with onlytop-side electrical contacts from which a flash silicone layer 16 hasbeen removed. Discrete light source 40 was manufactured using the methodof FIG. 5. Discrete light source 40 results from the segmentation of thearrays of LED dies 11 mounted on MCPCB 10 of FIG. 1. The printed circuitboard (PCB) segment 41 of discrete light source 40 has a top side 42, abottom side 43, and four edges 44-47. A light emitting diode die 48 isdisposed on the top side 42 of PCB segment 41. A contact pad 12 is alsodisposed on the top side 42 of PCB segment 41. A layer of silicone 16 isdisposed over LED die 48 and extends to each of the edges 44-47 of PCBsegment 41 except where the silicone flash layer 16 has been removedthrough blasting. The layer of silicone 16 is not disposed laterallyabove a portion of contact pad 12 at blasting site 15. In the embodimentof FIG. 10, the silicone flash layer 16 has not been removed from theentire surface of contact pad 12; a small portion of the trace layerthat forms contact pad 12 remains covered by silicone.

All of the electrical contacts on discrete light source 40 are on thetop side 42. Thus, PCB segment 41 has no electrical contacts on thebottom side 43. The layer of silicone 16 forms lens 20 above LED 48.There are less than three millimeters between the edge 46 of lens 20 andany of the edges 44-47 of PCB segment 41 because discrete light source40 was segmented from a high density printed circuit board 10. There arealso less than three millimeters between the edges 44-47 of PCB segment41 and any of the LED dies in the array of LED dies. There are no holesthat pass from the top side 42 to the bottom side 43 of PCB segment 41.Any punch-outs, through holes or etching cuts in the top side 42 ofdiscrete light source 40 would have hampered the formation of lens 20using compression molding because the pressurized molding material wouldhave escaped through the holes. The silicone at the edge of blastingsite 15 contains a trace amount of the blasting medium that remainsembedded in the silicone.

In another embodiment, water-based jetting is used to remove a flashlayer of silicone. Purified water is pressurized to a pressure ofbetween fifty and one thousand pounds per square inch and then forcedthrough a nozzle with an opening diameter between one hundred and onethousand microns. The exiting water beam is aimed directly at the flashlayer over the electrical contact pads until the flash layer is removed.The combination of the water pressure and nozzle diameter is chosen toachieve a stream of water with enough momentum to break the siliconeflash layer but yet that leaves the metal trace layer undamaged.Alternatively to using pure water, abrasive particles such as silica,aluminum oxide or garnet particles can be added to the stream of waterto allow a more efficient deflashing process at a lower water pressurecompared to with pure water.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A method, comprising: directing a flow of air ata top surface of a flash layer, wherein the flash layer is disposedabove a contact pad on a printed circuit board, and wherein the flashlayer is made of silicone; and colliding particles into the flash layer,wherein the particles are carried in the flow of air, and wherein theparticles are collided into the flash layer until the flash layerlaterally above the contact pad is removed.
 2. The method of claim 1,further comprising: forming the flash layer over the printed circuitboard using compression molding.
 3. The method of claim 1, wherein noholes pass from a top side of the printed circuit board to a bottom sideof the printed circuit board.
 4. The method of claim 1, wherein thecontact pad is located on a top side of the printed circuit board, andwherein no electrical contacts are disposed on a bottom side of theprinted circuit board.
 5. The method of claim 1, wherein the flow of airis directed at the top surface of the flash layer at an angle that iswithin thirty degrees of a normal angle to the top surface.
 6. Themethod of claim 1, wherein the flow of air is directed at the topsurface of the flash layer at an angle that is within forty-five degreesof a normal angle to the top surface.
 7. The method of claim 1, whereinthe flow of air is generated by compressing the air to a pressure ofmore than fifty pounds per square inch and allowing the compressed airto escape from a nozzle that has a diameter of less than twomillimeters.
 8. The method of claim 1, wherein the flow of air isgenerated by compressing the air to a pressure between fifty and twohundred pounds per square inch.
 9. The method of claim 1, wherein theflow of air emanates from a nozzle that is more than twenty millimetersfrom the top surface of the flash layer.
 10. The method of claim 1,wherein the particles have a median diameter that is between forty andsixty microns.
 11. The method of claim 1, wherein the particles are madeof sodium bicarbonate.
 12. The method of claim 1, wherein the flashlayer has a top side, a bottom side, and four edges, wherein the contactpad disposed on the top side of the printed circuit board, and whereinthe flash layer extends to each of the four edges of the printed circuitboard.
 13. The method of claim 1, wherein the silicone forms a lensabove a light emitting diode, and wherein there are less than threemillimeters between an edge of the printed circuit board and an edge ofthe lens.
 14. A method, comprising: positioning a nozzle within thirtymillimeters of a top surface of a layer of silicone; directing a streamof air that exits from the nozzle towards the top surface at an anglethat is between five and thirty degrees away from a normal angle to thetop surface; adding particles to the stream of air such that theparticles are carried by the stream of air and collide into the topsurface of the layer of silicone, wherein the layer of silicone isdisposed above a contact pad on a printed circuit board; and collidingthe particles into the layer of silicone until the layer of siliconelaterally above the contact pad is removed.
 15. The method of claim 14,further comprising: forming the layer of silicone over the printedcircuit board using compression molding.
 16. The method of claim 14,wherein the layer of silicone covers a light emitting diode.
 17. Themethod of claim 14, wherein the particles have a median diameter that isbetween forty and sixty microns.
 18. The method of claim 14, wherein theparticles are made of sodium bicarbonate.
 19. The method of claim 14,wherein the layer of silicone forms a lens above a light emitting diode.20. The method of claim 19, wherein the printed circuit board has anedge, and wherein there are less than three millimeters between the edgeof the printed circuit board and an edge of the lens.